1. Field of the Invention
The present invention relates to an image formation apparatus in a copying machine, facsimile, printer or the like, and in particular to an image density controlling device for a color image formation apparatus having a plurality of image formation means such as light-sensitive members for electrophotography, and performing an electrophotographic image formation process on the light-sensitive members to form a developed image of one or more colors, and transferring the image to a transfer member carried by a transfer member carrying means or an intermediate transfer member.
2. Discussion of the Related Art
The U.S. Pat. Nos. 2,576,882 and 3,357,325, for example, disclose an image formation apparatus wherein an insulating transfer belt or the like moving synchronously with an image formation means has a transfer sheet electrostatically adhered thereto and carries it to a transfer position of the image formation means, and transfers a toner image formed on the image formation means by an electric field on the reverse side of the insulating transfer belt to the transfer sheet.
One advantage of a method of transferring images to each of the transfer sheets by using the above-described insulating transfer belt as the transfer sheet carrying means, is that it has a sufficient transfer, charge capacity to counteract the increased toner weight of a plurality of superimposed toner images.
The image formation apparatus of this type comprises a first corona discharger for electrostatically attaching the transfer sheet to the transfer belt, a second corona discharger for generating an electric field on the reverse of the transfer belt by corona discharge for transfer of the toner image, a detachment corona discharger discharging for and detaching the transfer sheet from the transfer belt after transfer is finished, and an alternating corona discharger discharging the transfer belt.
As for the method of discharging the transfer belt, Japanese Patent Application Unexamined, Publications Nos. Sho. 63-195350 (1988), Sho. 63-195351 (1988) and Sho. 63-195352 (1988) suggest a method wherein the detachment corona discharger is disposed between the last transfer process and the first transfer process of the next cycle.
Since there is the possibility of contamination of the reverse side of the subsequent transfer sheet by toner attaching the surface of the transfer belt when the transfer sheets are jammed or a toner image formed on the light-sensitive member does not fit to the transfer sheet, a cleaner is provided to clean the surface of the transfer belt for carrying the transfer sheet.
Moreover, in the image formation apparatus forming a color image by using a plurality of light-sensitive members, registration errors of the transfer sheet in the direction of transportation are apt to occur. To prevent these errors, Japanese Patent Application Unexamined Publication No. Sho. 63-300263 (1988) proposes a method of transferring a pattern for correcting registration errors, reading the position of the pattern with a CCD or similar sensor, automatically registering the images of each color and cleaning off the pattern.
Japanese Patent Application Unexamined Publications Nos. Sho. 63-279275 (1988), 63-279276 (1988) and 61-53756 (1986) disclose a method of transferring a pattern for correcting registration errors, reading the position of the pattern by a photosensor or the like, and at the same time controlling the density of each color toner in accordance with the output of the photosensor.
Cleaning the surface of the transfer belt has been accomplished using a cleaning blade, fur brush, web or the like and combinations thereof.
There are some problems in the method of transferring the pattern for correcting the registration errors or a toner image patch for controlling the image density on the transfer belt and reading it with the photosensor, which are as follows.
In the image formation apparatus forming a color image by using a plurality of image formation means 1a, 1b, 1c and 1d shown in FIG. 1, the change of potential of a transfer belt 2 in each of the processes of transferring the toner image patches of different colors for controlling image density with a standard density of 70%, for example, to the surface of the transfer belt 2 one by one, detecting the image density by a photosensor 3 after the last image formation means 1d completes the transfer, discharging the transfer belt 2 by a detachment corona discharger 4, and removing the toner image patches on the surface of the transfer belt 2 by a cleaning means 5, are shown in FIG. 2.
In FIG. 1, 6 is a belt discharging means for discharging the transfer belt 2, 7 is a cleaning means for cleaning the transfer belt 2 and 8 is an attachment means, whose working areas are indicated as A, B and C, respectively, in FIG. 2. Transfer areas of the image formation means 1a, 1b, 1c and 1d are respectively indicated as D, E, F and G, and detaching means 4 and 5 are indicated as working area H in FIG. 2.
After passing through the working area G of the last (fourth) image formation means 1d, the surface of the transfer belt 2 has a charge of about (-) 4000 V, and after passing the belt discharge means 6 (through working area A), the charge of portions of the transfer belt surface on which no toner image patches for controlling the image density are formed or portions where the transfer sheets are attached is reduced to approximately (-) 100 V. However, as indicated by a broken line in FIG. 2, a charge of about (-) 400 to 600 V remains on portions where the toner image patches for controlling the image density are present in the working area A. Even after passing through the working area B of the cleaning means 7 and the working area C of the attachment means 8, the charge still remains. This is because the surface of the transfer belt 2 cannot be sufficiently discharged by applying an alternating corona discharge over the toner image patches, though the toner image on the surface of the transfer belt 2 can be discharged. As shown in FIG. 2, the influence of the residual charge gradually reduces in the subsequent processes of the image formation means, but still remains.
The transfer belt 2 is an insulating belt with a join. The part of the transfer belt 2 corresponding to the working areas of image transfer D, E, F and G is equally divided into an integral number of panels and the toner image patches are transferred for every cycle of the transfer belt rotation the same areas between the panels where no image transfer is carried out. FIG. 3 shows the state of discharge of the transfer belt 2 when the patch transfer process described above is repeated.
In FIG. 3, during the first cycle of the image formation, the charge on the transfer belt 2 is approximately 0 V because the whole belt is positively discharged in advance. During the second and subsequent cycles, a charge of (-) 300 to (-) 400 V remains on the portions of the toner image patches for controlling the image density transferred to the areas where no image transfer is carried out. The characteristics shown in FIG. 3 are under the conditions of normal temperature and humidity. FIG. 4 shows the characteristics in the low temperature and humidity, where the efficiency of the discharge is decreased and the residual charge is about (-) 500 to (-) 600 V.
If the transfer belt 2 with the residual charge of the above-described amount is advanced to the working area C of the attachment means 8 for the next transfer sheet and the subsequent working areas D, E, F and G of the image formation means, the difference in the residual charges between the portions of the toner image patches for controlling the image density and the portions without them is gradually reduced, but the residual charge at the first image formation means 1a becomes a maximum, as shown in FIG. 2.
FIG. 5 shows the relationship between the residual charge on the transfer belt 2 and the transfer efficiency of the toner in the working area D of the first image formation means 1a. As seen from the figure, the residual charge on the transfer belt 2 after passing through the belt discharge means 6 is more than (-) 200 V, and the transfer efficiency in the first working area D is severely reduced. The transfer efficiency is evaluated by the weight of toner transferred from the toner image patch.
As seen from FIG. 6, the density of the toner image patch also affects the variation of the residual charge on the transfer belt after passing through the detachment corona discharger 4. As the toner weight of the toner image patch increases, the amount of the residual charge on the transfer belt 2 becomes larger, and therefore it is preferable to restrict the toner weight of the toner image patch to approximately 0.6 mg/mm.sup.2 or less.